ImportanceA better understanding of the psychosocial health of resettled child and adolescent refugees and associated premigration and postmigration factors may help this population integrate effectively.ObjectiveTo estimate the associations of premigration and postmigration multidomain factors with psychosocial health after resettlement among young refugees of different ages.Design, Setting, and ParticipantsThis cross-sectional study used wave 3 data from the Building a New Life in Australia (BNLA) cohort study, as they represented the first time a BNLA study included a child module targeting children and adolescents in the migrating unit as a nested component of the broader study. The study population consisted of children aged 5 to 10 years and adolescents aged 11 to 17 years. The caregivers of the children, the adolescents themselves, and the adolescents’ caregivers were invited to complete the child module. Wave 3 data were collected from October 1, 2015, to February 29, 2016. Statistical analysis was performed from May 10 to September 21, 2022.ExposuresPremigration and postmigration multidomain factors, including individual (child and caregiver), family, school, and community levels, were measured.Main Outcomes and MeasuresSocial and emotional adjustment and posttraumatic stress disorder (PTSD) were the dependent variables measured by the Strengths and Difficulties Questionnaire (SDQ) and an 8-item PTSD scale. Weighted multilevel linear or logistic regression models were used.ResultsOf the 220 children aged 5 to 10 years (mean [SD] age, 7.4 [2.0] years), 117 (53.2%) were boys; of the 412 adolescents aged 11 to 17 years (mean [SD] age, 14.1 [2.0] years), 215 (52.2%) were boys. Among the children, compared with no exposure, exposure to premigration traumatic events (β = 2.68 [95% CI, 0.51-4.85]) and having family conflicts after resettlement (β = 6.30 [95% CI, 2.97-9.64]) were positively associated with SDQ total difficulties score; school achievement was negatively associated with SDQ total difficulties score (β = −5.02 [95% CI, −9.17 to −0.87]). Among the adolescents, being treated unfairly (β = 3.32 [95% CI, 1.41-5.22]) and parenting harshness after resettlement (β = 0.25 [95% CI, 0.11-0.40]) were positively associated with SDQ total difficulties score; engagement in extracurricular activities (β = −3.67 [95% CI, −6.83 to −0.50]) was negatively associated with SDQ total difficulties score. Exposure to premigration traumatic events (adjusted odds ratio [aOR], 2.49 [95% CI, 1.10-5.63]), being treated unfairly (aOR, 3.77 [95% CI, 1.60-8.91]), and facing English language barriers (aOR, 6.41 [95% CI, 1.98-20.79]) after resettlement were positively associated with the presence of PTSD.Conclusions and RelevanceIn this study of refugee children and adolescents, apart from premigration traumatic experiences, several postmigration family- and school-related factors and social integration factors were associated with psychosocial health after resettlement. The findings suggest that family- and school-centered psychosocial care and social integration programs targeting related stressors merit increased attention for improving the psychosocial health of refugee children and adolescents after resettlement.